The genes which determine resistance to mercuric ion (Hg(II)) are carried on many antibiotic resistance plasmids in both Gram-negative and Gram-positive bacteria. Plasmid-determined HgCl2-resistance is due to enzymatic reduction of Hg(II) to Hg(O) by the Hg(II) reductase. This reduction detoxifies the mercuric ion by converting it to a volatile, inert form. The Hg(II) reductase is an intracellular, flavoprotein which requires NADPH as a cofactor. This enzyme appears to be similar to the disulfide-oxidoreductases such as glutathione reductase and lipoamide dehydrogenase. The Hg(II) reductase (merA), and an Hg(II) uptake function (merT), are part of a mercury resistance (mer) operon which is inducible by sub-toxic levels of HgCl2. A positively acting regulatory function (merR) appears to be responsible for this induction. We have characterized the peptides encoded by the mer operon and have constructed preliminary physical and genetic maps of the operon using the techniques of both conventional and molecular genetics. We plan to correlate the physical and genetic maps of the operon through the use of in vitro mutagenesis of the operon DNA and biochemical and genetic characterization of the mutant derivatives. We are principally concerned with the analysis of the regulation of the operon. We will clone the gene for the trans-acting regulatory element, merR and study, at the molecular level, its interaction with merP, a promoter which we have already cloned from the operon. We will exploit mer-lac operon fusions to discover gratuitous inducers and anti-inducers of the operon. We will also specifically mutagenize the Hg(II) reductase structural gene and the structural genes for the Hg(II) uptake proteins in order to dissect their mechanism of action.